Adhesives may be broadly divided in two classes: structural and pressure-sensitive. To form a permanent bond, structural adhesives harden via processes such as evaporation of solvent, reaction with UV radiation (as in dental adhesives), chemical reaction (such as multi part epoxy), or cooling (as in hot melt). In contrast, pressure-sensitive adhesives form a bond simply by the application of light pressure to marry the adhesive with the adherend. Pressure-sensitive adhesives are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow, or wet the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in proximity, there are also molecular interactions such as van der Waals forces involved in the bond, which contribute significantly to the ultimate bond strength. PSAs exhibit viscoelastic (viscous and elastic) properties, both of which are used for proper bonding.
As the name suggests, pressure-sensitive adhesives are capable of bonding to surfaces simply by the application of light pressure. This makes them very convenient products and accounts for its usage in fields as varied as automotive, electronics and dental applications. Acrylic polymers have been most widely used as pressure-sensitive adhesives in commercial products because of the low-cost, nontoxicity, transparency, and good resistance to weathering.
Most conventional pressure-sensitive adhesives are designed for applications at room temperature and with multiple-components. However, the demand for single-component acrylic pressure-sensitive adhesives with high adhesive properties applicable at wide ranges of temperature has increased rapidly in recent years. It is well-known that tack (the ability of pressure-sensitive adhesives to be quickly adhered to an objective surface at a slight pressure), peel strength (the ability of pressure-sensitive adhesives to resist interface separation by peeling), and shear strength (the ability of pressure-sensitive adhesives to resist creep when shear force is applied up on) are the major parameters that define the end-use properties of pressure-sensitive adhesives. The values of these parameters are associated with and can be adjusted by intrinsic viscosity, glass transition temperature (Tg), molecular weight (Mw), and crosslinking density. Generally, modulating the crosslinking density is the most effective method to improve adhesive properties, especially for heat and chemical resistance. While those skilled in the art generally recognize that properties of pressure-sensitive adhesives can be adjusted, hese adjustments generally produce static results. Presently there is a need in the art for pressure-sensitive adhesives with controllable properties such as adhesive strength.